The interior of Mars’ moon Phobos could be as much as 30 percent empty space, new observations suggest. Though it’s still not clear how the object formed, the finding means it is probably not an asteroid that was captured by the Red Planet’s gravity, researchers say.

Scientists have long debated the origin of Phobos, and these new findings narrow down the possibilities, says Tom Andert, a planetary geophysicist at the University of the German Armed Forces in Munich. He and his colleagues re...